The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
The functional principle of a triangulation light sensor is based on the fact that light beams transmitted by a light source are scattered back onto a receiver by an object which has moved into its beam path, with the angle at which the light beams scattered back are incident on the receiver being a measure for the distance between the light sensor and the object. A triangulation light sensor with a circuit output generates an object determination signal when an object is located within a predetermined scanning range. A spacing-measuring triangulation light sensor can measure the distance between the light sensor and an object and deliver an output signal proportional to the distance.
The light source, which can additionally be provided with an optical transmitter system, emits an ideally parallel bundle of rays which generates a light spot when incident onto the surface of an object which is scattered back by the object. This light spot is imaged into a receiver plane via an optical receiver system arranged laterally offset to the light source, with the imaging as a rule taking place in a blurred manner since the receiver plane is usually not in the image plane. The beams scattered back by the object pass through the optical receiver system at different angles in dependence on the distance of the object from the optical receiver system and are thus also incident onto different lateral positions in the receiver plane. The spacing of the object from the light sensor can be determined from the spacing of the light spot imaged onto the receiver from a reference point by means of triangulation with knowledge of the spacing between the optical receiver system and the receiver plane as well as of the location and direction of the bundle of rays transmitted by the light source.
With an ideal triangulation light sensor, the optical receiver system is formed by an infinitely small aperture since a detection of the spacing independently of interference influences is only thereby ensured. With a real triangulation light sensor, an optical receiver system is used with an aperture whose minimal size is predetermined by the limited sensitivity of the receiver and by the signal-to-noise ratio of the total system.
With a real light sensor of this type, however, the measurement result can be falsified if only a portion of the optical receiver system is illuminated in an asymmetric manner. This can occur, for example, in that the light pencil transmitted by the light source is incident on a contrast border present in the surface of the object, with the contrast border separating regions with very high backscattering capability and regions with very low backscattering capability from one another. The measured light spot focus is thereby displaced in an unwanted manner with respect to the anticipated light spot focus in the receiver plane.
Furthermore, only objects with ideally scattering surfaces can be detected really reliably by triangulation light sensors, i.e. the object should scatter back an incident light beam uniformly diffusely at a specific spatial angle. Real surfaces are, however, rarely exclusively light scattering; as a rule, at least some of the incident light is reflected.
A light beam reflected by the object as a rule only partly illuminates the optical receiver system in contrast to a light beam scattered back. If the reflected bundle of rays is not incident on the optical receiver system in the region of the optical axis, a light spot is generated by the reflected beams whose position differs from the position of the beams scattered back. Since the reflected beams as a rule have a higher intensity than the beams scattered back, a false detection can occur in dependence on the angle of the object surface to the transmission direction of the light beams transmitted by the light source.